VERIFICATION OF SPENT FUEL ASSEMBLIES (MTR) IN STORAGE FACILITIES

摘要

The Central Store for Special Irradiated Fissionable Material (DCMFEI) is an old Argentine facility, designed to store LEU spent fuel assemblies discharged from the research reactors. Spent MTR fuels are normally stored inside concrete wells filled with water and closed with lead plugs on the top. The water is used as shielding and cooling media. Each well can accommodate up to two fuel assemblies. The facility is under the regulatory control of the Argentine Nuclear Regulatory Authority (ARN). On the other hand, the Brazilian-Argentine Agency for Accounting and Control of Nuclear Material (ABACC) and the International Atomic Energy Agency (IAEA) carry out safeguards inspections to this facility since the beginning of the implementation of the Quadripartite Agreement. The spent fuel assemblies are maintained under dual containment system by ABACC and IAEA, which requires the verification of fuel assemblies at research reactors and to maintain the continuity of knowledge during the transfers to the storage. The transfer campaign takes one week and it is carried out once per year. In order to reduce the inspection effort is proposed to remove all seals. In this case, the Safeguards Criteria for Storage Facilities request item counting and verification of the spent fuel for gross defect. Nevertheless, this task present some problems because the spent fuels are not always visible, then item counting is difficult. On the other hand, the introduction of detectors inside the wells is not always possible due to the small space available and the high doses involved when the wells remain opened. In order to overcame these difficulties ARN and ABACC have designed and tested a method to verify spent fuel assemblies using gamma spectrometry. Spectra were taken with a Cadmium Telluride detector (CZT). An ad hoc modified lead plug was used as detector collimator. The activity of each item was determined trough the main Cs-137 peak. Experience showed that it is possible to distinguish among one, two or no spent fuel assembly. Moreover, the activity measured on each well shows good correlation with the theoretical activity calculated considering burnup, decay time and shielding corrections.